Please explain how polarisation of waves occurs?

Electromagnetic waves can be imagined by a sinusoidal wave moving up and down, with this movement perpendicular to the direction that they are travelling in. Imagine a sea wave, it moves up and down but also forwards. When electromagnetic waves are produced in the real world, different waves will move up and down in different directions, even if they're moving forward in the same direction. Imagine lots of light waves moving into the page, and their sine waves form a circle around the point of light. Polarisation is when only a small selection of direction is allowed. So a polarising filter will block all directions of the circle except a few.